Among other things, coordinate measuring machines (“CMMs,” also known as surface scanning measuring machines) measure unknown surface profiles, or verify the topography of known surfaces. For example, a CMM may measure the topological profile of a propeller to ensure that its surface is appropriately sized and shaped for its specified task (e.g., moving a 24 foot boat at prespecified speeds through salt water). To that end, conventional CMMs typically have a probe head supporting a probe tip (also called a “stylus”) that directly contacts and moves along a surface of an object being measured.
After the tip initially contacts the surface of an object, the probe head is driven toward the object surface to dampen vibrations and to prepare it for subsequent scanning (often referred to as “settling” the probe head). After settling, the probe tip moves along a predefined scan path, recording its movement in one, two, or three dimensions. Logic associated with the CMM records this movement to produce a corresponding surface topological map of the measured object.
During a typical scan, the CMM may move the probe tip into and out of contact with the surface many times to avoid known deep recesses or other surface features. Both initial and final contact with the surface, however, create a number of performance challenges. Specifically, upon initial approach to the surface, the CMM has competing goals of approaching as quickly as possible, while at the same time avoiding a hard landing. Many conventional CMMs manage this by sacrificing speed—they slowly approach a surface along a normal vector, stopping both before and after contact with the surface. This approach thus reduces throughput, increasing costs.
Another technique brings the tip into contact with the surface along a very shallow angle with the surface, and appears to settle while moving at that shallow angle. While attempting to mitigate the hard landing issue, this technique still is time consuming because of the lengthy time it takes to approach the surface. In addition, the shallow angle makes the settling more challenging.